Compat for old assets

A couple times this release, I saw reports of old assets before broken for various reasons. One was .NET Core 2.2 and the other for .NET Core 3.1.

Our primary policy should be that people can use old SDKs for unsupported .NET versions. They exist and will provide the best compat.

I'd like to propose a new policy for test assets: **we maintain test assets for two LTS version back**

This means:

- For .NET 10, we'd test with .NET 6+ assets
- For .NET 11/12, we'd test with .NET 8+ assets
- Older assets should be deleted.
- It's fine to keep older assets for one-off manual testing, but not for anything automated.

Even spending one minute looking at failures for .NET Core 2.2 apps is a poor use of time. In that particular case, the underlying reason was "resolved" by a change in design in .NET Core 3.1.